1. Field of the Invention
The present invention relates to a dot-line printer. Such printers have a hammer bank consisting of a plurality of dot-printing hammers separately provided at intervals along a printing line. The hammer bank is reciprocated along the printing line to thereby carry out the printing of characters along the printing line. A space of several dot-lines separates adjacent printed lines of characters.
2. Background of the Invention
FIG. 1 is a schematic of a conventional dot-line printer. The printer includes hammer bank 10 consisting of a plurality of dot-printing hammers 12 juxtaposed along printing positions. Means (not-shown) for driving the printing hammers 12 is reciprocated by a shuttle mechanism 16 along the printing positions parallel to the axial direction of a platen 2. The shuttle mechanism is conventionally a cam, a linear motor or the like. The printing hammers 12 may be any of several known constructions such as, for example, the spring charge type having a leaf spring and a printing pin 14 attached to the upper end of the leaf spring as shown in FIG. 2. The leaf spring is held by a permanent magnet or the like at a non-printing position. When the leaf spring is released it moves paper. As will be made clear hereinafter, the invention, however, is not limited to such a spring charge type printing hammer and any kind of printing hammer may be employed. An ink ribbon 6 is arranged to run between the platen 2 and the hammer bank 10 along the printing positions. The paper 8 to be printed is fed between the platen 2 and the ink ribbon 6 in the direction perpendicular to the running direction of the ink ribbon 6 by means of a pair of paper feed tractors.
Printing is carried out by driving desired printing hammers 12 while the hammer bank 10 is reciprocated. The reciprocation that is, the forward and backward movement of the hammer bank is called "shuttle movement", and one shuttle movement corresponds to a single forward or backward movement of the hammer bank in the reciprocation process.
The manner in which a dot-line printer prints characters will be explained with reference to FIG. 3. FIG. 3A illustrates the printing of the Chinese character "KAN" by the hammers 12. At the top of FIG. 3A is illustrated two adjacent printing pins 14 and 14' separated by an interval of 0.15 inches. The left printing pin 14 is moved right by the shuttle movement of the hammer bank 10 so as to carry out printing in the region bounded on the right by the illustrated position of the right printing pin 14'. Similarly, the right printing pin 14' carries out printing in a region beginning to the right of printing pin 14'.
A matrix having 1/180-inch vertical and horizontal intervals shows points on which the printing pins 14 carry out printing. Hereinafter, the horizontal and vertical positions are referred to as dot-positions and dot-lines, respectively. In the horizontal direction, each of the printing pins 14 carries out printing on the matrix from the first dot-position to the 27th dot-position. In the vertical direction, one printed line is usually 1/6 inches wide, so that the first printed line is comprised of the first dot-line to the 30th dot-line. Printed lines each having 30 dot-lines are successively printed. In the case of printing characters, printing is carried out from the first dot-line to the 24th dot-line of the 30th dot-lines for printing a character line. The portions from the 25th dot-line to the 30th dot-line are defined as the interline space and contain no printing.
FIG. 3B shows the locus of points drawn by a printing pin 14 on the paper 8 in response to the reciprocation of the hammer bank 10 and the paper feed movement. In FIG. 3B, the lefthand numerals represent the shuttle movement number in the printing of each line, and the righthand numerals represent the number of dot-lines printed during the shuttle movement. Printing is actually carried out in a printing period of each shuttle movement. During the printing period the paper 8 is stationary. No printing occurs in a shuttle direction reversing period. However, during the shuttle direction reversing period paper 8 is fed to the next dot-line.
As is apparent from FIGS. 3A and 3B, the hammer bank 10 performs shuttle movement 24 times to print 24 dot-lines from the first dot-line to the 24th dot-line. During the 25th shuttle movement no printing occurs and the paper is fed over 7 dot-lines from the 25th dot-line to the printing start position (i.e., dot-line) of the next character line; that is, to the first dot-line of the second character line. The time necessary to feed the paper 8 over 7 dot-lines is longer than the time necessary to feed the paper one dot-line. Therefore, the shuttle direction reversing period between adjacent dot-lines of a character line used to feed the paper one dot-line is too short to feed the paper over 7 dot-lines. For this reason, the 25th shuttle movement is allotted to the time period for feeding the paper over 7 dot-lines. That is, in order to print one character line, 25 shuttle movements are required; 24 shuttle movements for actually printing the character and one shuttle movement for a 7 dot-lines feed.
The period of time in which the dot of the printing hammer 12 can be successively printed; that is, the hammer repeatability, is about 0.5 ms. The time necessary for feeding paper between successive dot-lines is 3 ms. Accordingly, the time required for printing one character line, which includes the paper, feeding time, is about (0.5.times.27+3).times.25=412.5 ms, so that the printing speed is about 145 lines per minute.
As is apparent from the above description, in order to increase the printing speed, it is necessary to improve the hammer repeatability and to shorten the paper feeding time. In order to improve hammer repeatability and shorten paper feeding time, however, many problems must first be solved so that such improvement is very difficult to effect.
One proposed solution to increasing printing speed involves increasing the number of printing hammers 12. FIG. 4 shows an example of such an arrangement. Two hammer banks 10 are provided on upper and lower stages respectively, such that the printing pins 14 of printing hammers 12 extending from each of the hammer banks 10 are aligned side by side at intervals of 0.075 inches. The amplitude of the shuttle movement of each of the hammer banks 10 becomes 1/2 of that of the single bank described above. That is, each print hammer covers 14 dot-positions. Accordingly, in this arrangement, the time required for printing one line is about (0.5.times.14+3).times.25=250 ms, so that the printing speed is about 240 lines per minute. In the arrangement of FIG. 4, one character is printed by two printing pins 14.
In the arrangement of FIG. 4, each of the hammer banks 10 is reciprocated at a very high speed such as about 50 Hz. Since the mass of the reciprocating hammer banks 10 is twice that of the single bank arrangement, the load applied to the shuttle mechanism 16 is increased, thereby increasing vibration and noise. Therefore, disordering of the printed dot-positions occurs to deteriorate the printing quality. Although the rigidity of the shuttle mechanism 16 may be heightened to prevent such disorder from occurring, the strengthening of the shuttle requires that it be large and costly to manufacture.